Semiconductor chips have gained widespread acceptance in a wide range of technical fields. However, they are still highly complex and expensive to produce. Although silicon substrates can be thinned to very low layer thicknesses, so that they become flexible, these processes are likewise very expensive, meaning that flexible or curved microchips are only suitable for demanding applications for which high costs can be accepted. The use of organic semiconductors offers the option of inexpensive fabrication of microelectronic semiconductor circuits on flexible substrates. One example of a possible application is a thin film with integrated control elements for liquid crystal displays. A further possible application is transponder technology, in which, for example, information about a product is stored on what are known as tags.
Field-effect transistors are used as switches in electronic circuits. In this case, in the off state of the transistor, a semiconductor that is arranged between a source electrode and a drain electrode, which are each composed of an electrically conductive material, acts as an insulator, while under the influence of the field of a gate electrode a charge carrier channel is formed in the on state of the transistor. In this case, electrical charge carriers are injected into the semiconductor layer at the source contact and are extracted from the semiconductor layer at the drain contact, so that an electric current flows from source to drain through the semiconductor layer or through the charge channel produced in the semiconductor layer.
On account of the different Fermi levels of semiconductor material and contact material, an asymmetric charge carrier diffusion process occurs at the contact surface between the two materials. On account of the different energy of the Fermi levels of the two materials, there is an energy difference which is compensated for by the transfer of charge carriers. As a result, an interface potential is built up which, when an external potential difference is applied, counteracts a transfer of the charge carriers between the two layers. Therefore, the result is a potential barrier which has to be overcome by the charge carriers when entering the semiconductor material from the electrically conductive contact or when leaving the semiconductor material to pass into the electrically conductive contact. The higher or wider the potential barrier, the lower the tunneling current which is formed as a result of the charge carriers tunneling through the potential barrier. A low tunneling current corresponds to a high contact resistance. In the case of semiconductor components based on inorganic semiconductors, an increase in the contact resistance is combated by doping the inorganic semiconductor in a boundary layer oriented toward the contact surface. The doping changes the energy of the Fermi level in the inorganic semiconductor, i.e. the difference between the Fermi levels of contact material and semiconductor material is reduced. The result is either a reduction in the potential barrier, allowing a significantly greater number of charge carriers to overcome the potential barrier and to swamp the opposite material, or a narrowing of the potential barrier, which increases the probability of charge carrier tunneling through the potential barrier. In both cases, the contact resistance is reduced.
For the fabrication of field-effect transistors based on amorphous or polycrystalline silicon layers, the contact regions are doped by the introduction of phosphorus or boron into the layer of silicon close to the source and drain contacts. The phosphorus or boron atoms are incorporated in the silicon network and act as charge donors or charge acceptors, with the result that the density of the free charge carriers and therefore the electrical conductivity of the silicon is increased in the doped region. This reduces the difference between the Fermi levels of contact material and doped semiconductor material. The doping substance is in this case introduced into the silicon only in the region of the source and drain contacts but not in the channel region, in which a charge carrier channel is formed under the influence of the field of the gate electrode. Since phosphorus and boron form covalent bonds with the silicon, there is no risk of these atoms diffusing into the channel region, and consequently a low electrical conductivity continues to be ensured in the channel region.
If the contact regions are sufficiently highly doped, the tunneling probability even in the quiescent state is so high that the junction between the contact material and the inorganic semiconductor material loses its blocking capacity and applies a good conductivity in both directions.
Field-effect transistors based on organic semiconductors are of interest for a wide range of electronic applications which require extremely low manufacturing costs, flexible or unbreakable substrates or require transistors and integrated circuits to be fabricated over large active surfaces. By way of example, organic field-effect transistors are suitable as pixel control elements in active matrix displays. Displays of this type are usually produced with field-effect transistors based on amorphous or polycrystalline silicon layers. The temperatures of usually more than 250° C. which are required to fabricate high-quality transistors based on amorphous or polycrystalline silicon layers require the use of rigid and fragile glass or quartz substrates. On account of the relatively low temperatures at which transistors based on organic semiconductors are fabricated, usually of less than 100° C., organic transistors make it possible to produce active matrix displays using inexpensive, flexible, transparent, unbreakable polymer films, which is associated with considerable advantages compared to glass or quartz substrates.
A further application area for organic field-effect transistors lies in the fabrication of very inexpensive integrated circuits, as are used for example for the active labeling and identification of goods and products. These transponders, as they are known, are usually produced using integrated circuits based on single-crystalline silicon, which entails considerable costs in construction and connection technology. The production of transponders on the basis of organic transistors would lead to enormous cost reductions and could help transponder technology to achieve a world-wide breakthrough.
One of the main problems of using organic field-effect transistors is the relatively poor electrical properties of the source and drain contacts, i.e. their high contact resistances. The source and drain contacts of organic transistors are generally produced using inorganic metals or with the aid of conductive polymers, in order in this way to ensure the highest possible electrical conductivity of the contacts. Most organic semiconductors which are suitable for use in organic field-effect transistors have very low electrical conductivities. For example, pentacene, which is often used to fabricate organic field-effect transistors, has a very low electrical conductivity of 10−14 Ω−1 cm−1. If the organic semiconductor has a low electrical conductivity, there is a considerable difference between the Fermi levels of electrically conductive contact material and organic semiconductor material at the contact surface. This leads to the formation of a high potential barrier with a low tunneling probability for the passage of charge carriers. Therefore, source and drain contacts often have high contact resistances, and consequently high electric field strengths are required at the contacts for charge carriers to be injected and extracted. Therefore, it is not the conductivity of the contact itself, but rather the low conductivity of the semiconductor regions which adjoin the contacts and into which the charge carriers are injected and from which the charge carriers are extracted which constitutes a limitation.
To improve the electrical properties of the source and drain contacts, therefore, it is desirable to achieve a high electrical conductivity of the organic semiconductor in the regions which adjoin the contacts, in order to reduce the difference in the Fermi levels between organic semiconductor and contact material and thereby to lower the contact resistances. On the other hand, a high electrical conductivity of the organic semiconductor in the channel region has an adverse effect on the properties of the transistor. A significant electrical conductivity in the channel region inevitably leads to high leakage currents, i.e. to relatively high electric current intensities in the off state of the field-effect transistor. For many applications, low leakage currents in the region of 10−12 A or below are imperative. Moreover, a high electrical conductivity leads to the ratio between the maximum switch-on current and the minimum switch-off current becoming too low. Many applications require the ratio between switch-on current and switch-off current to be as high as possible, in the region of 107 or above, since this ratio reflects the modulation behavior and the gain of the transistor. Therefore, a low electrical conductivity of the organic semiconductor is required in the channel region, while a high electrical conductivity is required in the region of the source and drain contacts, in order to improve the contact properties between organic semiconductor material and the material of the contacts.
As with inorganic semiconductors, the electrical conductivity of many organic semiconductors can be increased by the introduction of suitable doping substances. However, there are problems with obtaining positional selectivity during doping. The doping substances are not bound to a specific position in the organic semiconductors and can move freely within the material. Even if the doping process can originally be restricted to a specific region, for example the regions around the source and drain contacts, the doping substances subsequently migrate through the entire semiconductor layer, particularly under the influence of the electric field which is applied between the source contact and the drain contact in order to operate the transistor. The diffusion of the doping substance within the organic semiconductor layer inevitably increases the electrical conductivity in the channel region.
I. Kymissis, C. D. Dimitrakopoulos and S. Purushothaman, “High-Performance Bottom Electrode Organic Thin-Film Transistors” IEEE Transactions on Electron Devices, Vol. 48, No. 6, June 2001, pp. 1060–1061, describe a semiconductor device with a reduced contact resistance, in which first of all a monomolecular layer of 1-hexadecanethiol is applied to chromium/gold electrodes, and then a layer of pentacene as organic semiconductor material is applied to the monomolecular layer of 1-hexadecanethiol. This arrangement makes it possible to significantly reduce the contact resistance to charge transfer of the charge carriers between electrode and semiconductor path. The molecules of 1-hexadecanethiol which are arranged at the interface between contact and organic semiconductor act as charge transfer molecules. They are in direct contact with both the contact material and the organic semiconductor layer. On account of their molecular structure, the charge transfer molecules can force a transfer of charge carriers between the contact material, in which there is an excess of charge carriers, and the organic semiconductor layer, in which there is a deficit of charge carriers. In this way, in the region of the source and drain contacts an excess of charge carriers can be produced in the organic semiconductor layer, with the result that the contact resistance is significantly reduced. The thiol groups of the 1-hexadecanethiol form a covalent bond with the surface of the gold contacts, resulting in local fixing of the molecules. Therefore, even under the action of a field applied between source and drain electrodes, the charge transfer molecules do not migrate in those sections of the organic semiconductor path in which the channel region is formed.
However, gold has the drawback of generally bonding very poorly to inorganic layers, such as for example to silicon dioxide. To improve the bonding of the gold contacts, therefore, a thin film of chromium or titanium as a bonding agent is often applied immediately before the deposition of the layer of gold. However, this has the drawback of making the patterning of the metal layer which is required in order to produce the contact structures more difficult. Furthermore, thiols are also only suitable as charge transfer molecules for certain metals, such as gold, since it is not possible to achieve a sufficient bonding strength to all metals which are suitable for the production of contacts to prevent the thiols from diffusing out of the boundary layer between contact and semiconductor material.